I think Snoke is either this guy (although I don’t watch the Rebels show, so maybe it’s completely impossible, dunno) or Darth Plagueis (using mainly the OST as evidence).

However, since I am not a big fan of the character so far (the name, the design, the role,…) I would love it if Snoke was in fact an illusion created by someone to manipulate the First Order to do the dirty work. There are already rumours that Benicio Del Torro could be playing a Thrawn-like character, so maybe he could be the one pulling the strings? It is very unlikely they would go this route, but it would make for a great twist while fixing one of TFA’s biggest drawbacks.